Tremolo circuit is a kind of sound effect. We can see this type of effect is applied  in guitar effect pedals.  Tremolo effect is produced if we modulate the amplitude of an audio signal. The shape of modulating signal can vary from square wave, sawtooth, or sine. When we use a square wave, then the effect produce a knocking-like sound which might be perceived like a percussive instrument. When we use sine wave then we might perceive a kind of vibrato effect which gives impression that there is a frequency modulation, but it is actually a  psycho effect because it modulate only the amplitude, not the frequency.  This tremolo circuit use very low frequency (VLF) as a source of modulating signal,  with frequency control by R12. The effect’s level, which is modulation depth,  is controlled by R11. The amplitude modulation is obtained by controlling the gain of U1D, which has feedback path through Q1 which receives signal from U1C phase oscillator. Here is the schematic diagram of the circuit:

Tremolo Circuit circuit schematic